Melt-blowing process comprises extruding a molten polymer through orifices, attenuating the extrudates into fibers by action of high-temperature high-speed gas that blows from near the orifices and collecting them on a belt conveyer comprising a wire net or the like, thereby forming a nonwoven fabric. This process is known to be capable of directly producing nonwoven fabrics comprising microfine fibers that cannot be produced by other processes. One of the features of the melt-blowing process is to extrude a polymer with its melt viscosity being about one order lower than that employed upon conventional melt spinning of general-purpose fibers. It is then become necessary either to use a polymer having a lower degree of polymerization than those used for conventional melt spinning or to elevate the temperature of the polymer being extruded. Any polymer satisfying the above conditions and having threadability, i.e. fiber formability, can be used for producing meltblown nonwoven fabrics. There are thus currently produced meltblown nonwoven fabrics comprising various polyolefins, polyamides, polyesters, polyurethanes or the like. There is, however, almost no production of meltblown fabrics comprising polyethylene terephthalate (hereinafter referred to as "PET"), which is a representative of polyesters and generally has advantages in view of good qualities and low cost.
This is because of low crystallization rate of PET as compared with other crystalline polymers being used for meltblown fabrics. When extruded under the usual melt-blowing conditions, PET does not increase the crystallinity sufficiently, although it can be attenuated into fibers with no problem. Then, the resulting fibers have low thermal stability and, when placed under relaxed condition at a temperature exceeding 70.degree.-80.degree. C., i.e. one exceeding the glass transition temperature of the polymer, shrink to a large extent, which is a very serious problem for practical purposes. Japanese Patent Application Laid-open No. 45768/1991 proposes to solve the above problem a process which comprises appropriately heat treating under tension the web having been meltblown and collected on a belt conveyer, thereby appropriately increasing the crystallinity. This process, however, requires an additional heat treatment step and, at the same time, yields a nonwoven fabric having lower strength and being more rigid than other meltblown nonwoven fabrics from conventional readily crystallizable polymers. This is considered to be due to the tendency of meltblown polyethylene terephthalate to generate spherulites.
Even with PET, employment of very specific conditions realizes production of a web having an areal shrinkage of not more than 10% in spite of the constituting fiber having crystallized to a low crystallinity, as described in Japanese Patent Application Laid-open Nos. 90663/1980 and 201564/1989.
Thus, Japanese Patent Application Laid-open No. 90663/1980 discloses a process which comprises blowing high-pressure air (1.5 to 6 kg/cm.sup.2) through an air gap having a narrow clearance of 0.2 mm or so. The process further comprises permitting the crystallization of the polymer leaving the orifice to progress by maintaining its intrinsic viscosity [.eta.] at least 0.55, preferably at least 0.6. To this end, it is necessary to extrude the polymer at a viscosity (at least 500 poises) considerably higher than the melt viscosity range that assures good melt-blowing condition of the polymer. The PET meltblown fabric thus obtained has good properties, such as strength, hand and thermal resistance. In commercial production with a machine having a width of at least 1.5 m, it is, however, difficult to maintain such a narrow gap clearance of less than 0.3 mm uniform throughout the machine width. There would occur uneven air blow widthwise, thereby generating uneven attenuation of polymer extrudates and further variation of secondary air flow accompanying the extrudates. As a result, there occurs in the web collected on a belt conveyer a continuous weight unevenness that resembles a wind-wrought pattern on the sand so that it becomes difficult to continue the operation.
In addition, the high pressure of at least 1.5 kg/cm.sup.2 of the primary air produces a large cooling effect due to adiabatic expansion. Then the PET extrudates are readily cooled and the high melting point of PET makes it difficult to produce pseudo-adhesion between the microfibers that formed. Consequently, the microfibers being collected onto the conveyer tend to scatter so that the collecting operation becomes unstable. This tendency becomes more marked with increasing polymer throughput per orifice and increasing volume of the primary air. Furthermore, with the conditions of high single orifice throughput under nigh pressure and high viscosity, shots (polymer particles) and nozzle soiling increase so that it becomes difficult to continue a long-period stable operation. To avoid this problem, a low throughput condition (0.7 to 0.2 g/orifice.multidot.minute) is necessarily employed, which lowers the productivity.
The process disclosed in Japanese Patent Application Laid-open No. 201564/1989 comprises jetting high-pressure secondary air through a narrow gap having a clearance of not more than 0.2 mm, and further using a long chamber for orientation having a length of at least 1 meter. Accordingly, this process also encounters large difficulty upon practicing with a large-width equipment on an industrial scale.
Under the above-described circumstances, no PET meltblown nonwoven fabrics are commercially produced today and costly polybutylene terephthalate, which has high crystallization rate and is hence free from the above difficulties, is used, as an only representative polyesters, for producing meltblown nonwoven fabric.
Japanese Patent Application Laid-open No. 99058/1985 proposes a process which comprises melt blowing PET in combination with another polymer. In this process, PET and PP are separately melted at different temperatures and then joined at the spinneret part, thereby forming microfine side-by-side composite fiber. In the usual melt spinning of general-purpose fiber, it is relatively easy to provide an equipment capable of joining 2 polymer flows at the spinneret part. With spinnerets for melt-blowing purpose, which must include passages for blowing air and arrange orifices in substantially one line only, provision of such joining device however renders the entire spinning head too complex so that the number of orifices should be extremely reduced, thereby decreasing the productivity. Furthermore, the microfiber obtained by this process is, like those in meltblown fabrics comprising PET only, not provided with increased crystallization rate. As a result, the thermal stability of such fiber is not improved.
In view of the above problems, the present inventors have made intensive studies to obtain, using PET, stably and efficiently, a meltblown nonwoven fabric having the excellent properties of PET, and completed the invention .